Microwave heating apparatus

ABSTRACT

The present invention relates to an apparatus for heating a sample, such as chemical reaction mixtures, whose dielectric properties varies during the heating process. In particular, the present invention relates to a microwave heating apparatus comprising a microwave generator, a waveguide for guiding the generated microwaves to an applicator, and a deflector formed by a closed loop defining a plane, said deflector having an inherent resonance frequency and a thickness in a direction normal to said plane, the deflector being rotatable around an axis being at least substantially parallel to said plane, the deflector being positioned in the waveguide so as to form a resonant cavity with the sample and the waveguide applicator. The resonance conditions of the resonant cavity and the coupling factor of radiation from the waveguide to the cavity are easily adjustable by rotation of the deflector. The resonance conditions and the coupling factor can be adjusted in response to the dielectric properties of the sample in order to optimise the amount of absorbed power and thereby obtain control of the sample heating process.

FIELD OF THE INVENTION

[0001] The present invention relates to an apparatus for heating asample, such as chemical reaction mixtures, whose dielectric propertiesvaries during the heating process. In particular, the present inventionrelates to a microwave heating apparatus comprising a resonant cavity inwhich the resonance conditions and the coupling factor of radiation tothe cavity are easily adjustable. The resonance conditions and thecoupling factor can be adjusted in response to the dielectric propertiesof the sample in order to optimise the amount of absorbed power andthereby obtain control of the sample heating process.

BACKGROUND OF THE INVENTION

[0002] One of the major obstacles for an organic chemist today is thetime consuming search for efficient routes in organic synthesis. Thechallenges for the pharmaceutical industries and the organic chemistinclude identification of ways of reducing time in drug development,identification of ways of creating chemical diversity, development ofnew synthesis routes and maybe reintroduction of old “impossible”synthetic routes. Also, it is a constant challenge to reach classes oftotally new chemical entities.

[0003] Chemical reactions are often performed at elevated temperature toenhance the speed of the reaction or supply enough energy to initiateand maintain a reaction. Microwaves assisted chemistry offers a way toperform reaction processes and circumventing at least some of theabove-mentioned problems, namely

[0004] speeding up the reaction time with several orders of magnitudes,

[0005] improving the yield of chemical reactions,

[0006] offering higher purity of the resulting product due to rapidheating and thereby avoiding impurities from side reactions, and

[0007] performing reactions that are not possible with conventionalthermal heating techniques.

[0008] Recent developments have lead towards apparatuses comprising amicrowave generator, a separate applicator for holding the sample to betreated, and a waveguide leading the generated microwave radiation fromthe generator and coupling it into the applicator. Even if the systemconsists of a 2450 MHz, TE₁₀ waveguide to which a magnetron generator isconnected in one end and the sample container is in the other end, thereis a need for a matching device in the form of at least a metal post oriris between the generator and load, in order to achieve a reasonableefficiency.

[0009] When coupling electromagnetic radiation such as microwaves from asource to an applicator, it is important to match the waveguideimpedance and the applicator impedance (with sample) in order to achievea good transfer of power. However, the dielectric properties of thesample will influence drastically upon the impedance of the applicator,as well as its electrical size, and the dielectric properties of thesample often change considerably with both temperature and appliedfrequency. Thus, an impedance mismatch between the source and theapplicator will often occur and the coupling and thereby the heatingprocess becomes less efficient and difficult to predict

[0010] U.S. Pat. No. 5,837,978 discloses a microwave heating systemapplying a resonant multimode applicator comprising means for impedancematching during a heating process in order to achieve resonance of thesystem. The matching or tuning is carried out by adjusting the height ofthe applicator and the position of a microwave antenna/probe in theapplicator (see e.g. column 7, lines 17-24 or column 8, lines 33-39).

[0011] In multimode cavities, the electric field is a superposition ofseveral longitudinal modes and several transverse modes. When amultimode applicator is tuned to resonance, one changes the balancebetween these modes and thereby the spatial energy distribution. Theenergy distribution is therefore neither spatially uniform nor constantduring the heating process, which makes it difficult to obtainreproducible results since a small change of the position or size of thesample, or a resonance tuning (performed by the user or by a change inthe dielectric properties of the sample), will resultin different powerabsorption. Rotation of the sample in the oven does not significantlyimprove the reproducibility, since some of the modes, as a matter offact most of the modes in a true multimode system, have a tendency toheat the outer parts of the sample more strongly. This leads to aposition dependent heating of the sample, which is also dependent uponthe resonance tuning. The samples used in microwave chemistry typicallyhave volumes ranging from a few μL to ˜10 mL, and it is thereforecrucial to have a uniform and known energy distribution.

[0012] WO 99/17588 discloses a microwave oven having a conductive memberfor controlling the feeding of microwave power from a waveguide to amultimode applicator. The conductive member acts as a diffractingresonator and provides a local region with a particular field pattern.When the member is rotated, the field changes, giving rise to anadvantageous feeding of microwave power to the multimode applicator. Theconductive member is preferably an elliptic ring member.

[0013] EP 552 807 A1 discloses a similar microwave oven having arotatable metal reflector in a waveguide for impedance matching betweenthe waveguide and a heating chamber.

[0014] Single mode applicator resonators offer a possibility of highfield intensities, high efficiency and uniform energy distributions. Theuse of single mode applicators have been reported, see e.g. US 5,393,492and US 4,681,740. However, since the dielectric properties of the samplechanges the resonance frequency and since magnetrons usually onlyprovide a fixed frequency or only a minor adjustment around the centrefrequency of the magnetron, the generated frequency and the resonancefrequency of the mode will detune as the sample heats. Thereby the highintensity in the resonant mode is lost.

[0015] US 2,427,100 and NL Octrooi No. 75431 both discloses means foradjusting the point impedance, or wave reflection, in microwavewaveguide transmission systems by having a conducting deflectorrotatably mounted in the waveguide. Both systems tune the waveguidesystem by introducing a reactance into the waveguide. Note that only thescattering, i.e. reflection of a specific waveguide mode, is affected.

[0016] U.S. Pat. No. 4,777,336 discloses a method for controllingheating patterns in single or multimode applicators by tuning theapplicator using a probe or sliding shorting plates within theapplicator.

[0017] It is generally a disadvantage of the multimode applicators thatthe spatial energy distribution changes when it is tuned for impedancematching.

[0018] It is another disadvantage of the multimode applicators that theapplicator has a non- uniform energy distribution.

[0019] It is a further disadvantage of the multimode applicators thatthe multimode heating pattern is not reproducible (i.e. very sensitiveto its dimensions) and may change as a function of the temperature ofthe load.

[0020] It is a disadvantage of the prior art single mode applicatorapparatuses that there are no efficient and durable means for tuning theresonance frequency in response to the dielectric properties of theload, since galvanic contacting by for example screw posts or metalvanes is needed for efficient control of also small coupling factors andthe air distances to the waveguide walls tend to become so small thatthere is a risk of arcing.

SUMMARY OF THE INVENTION

[0021] In view of the foregoing, an object of the present invention isto provide a microwave heating apparatus wherein the samples can beuniformly heated by using a single mode applicator.

[0022] Another object of the present invention is to provide a microwaveheating apparatus that has a high efficiency in that the coupling ofradiation to a sample held in the applicator is improved.

[0023] Still another object of the present invention is to provide amicrowave heating apparatus wherein coupling to a single mode applicatorand a resonance frequency of the applicator can be adjusted in responseto variations in dielectric properties of a sample in the applicatorusing a single rotatable deflector.

[0024] In a first aspect, the present invention provides a heatingapparatus comprising:

[0025] generating means for generating electromagnetic radiation at awavelength ,

[0026] a waveguide for guiding the generated electromagnetic radiationto a waveguide applicator for holding a sample to be heated, the samplehaving dielectric properties ε_(sample) which varies as a function of atemperature of the sample, the waveguide and the waveguide applicatorsupporting a single normal transverse mode,

[0027] a deflector formed by a closed loop defining a plane, saiddeflector having a resonance frequency ν_(defl) and a thickness in theinterval [λ/₃₀; λ/₅] in a direction normal to said plane, the deflectorbeing rotatable around an axis being at least substantially parallel tosaid plane,

[0028] the deflector being positioned in the waveguide so as to form aresonant cavity with the sample and the waveguide applicator, saidcavity having at least one resonance frequency ν_(cav) being dependentupon at least ε_(sample), ν_(defl), and an angle of rotation of thedeflector, α_(defl).

[0029] In the present context, waveguide should be interpreted as anymeans capable of guiding electromagnetic waves such as electromagneticradiation. The waveguide may be a waveguide in the form of metallicchannels for guiding waves such as radiation or cables such as coaxialcables for guiding waves such as electrical signals. The waveguide mayalso comprise active and/or passive components such as couplers,dividers, splitters, combiners, circulators, power meters, artificialsamples, spectrum analysers etc.

[0030] The waveguide may typically support only a single transversemode, TE or TM, depending upon its design. The waveguide is preferablyconnected to the applicator so as to transfer energy from modes in thewaveguide to modes in the applicator. In order for the coupling to beefficient, the impedance of the waveguide must be at least substantiallymatched with the impedance of the applicator, and there may also be afield matching (i.e. possibility of continuous energy transfer by fieldsimilarities in the two guides). The coupling of radiation, and hence ofenergy, from modes in the waveguide to modes in the applicator can,under conditions of field matching, be quantified by the coupling factordefined as the ratio between the impedance of the waveguide and theimpedance of the applicator. It is typically desirable to have as goodan impedance matching as possible (or equivalently, a coupling factor asclose to 1 as possible) under the actual conditions. This impedancematching (or coupling factor optimisation) may be obtained underdifferent conditions depending on different parameters such as theabsorbency of the sample and the design of the system. When rotating thedeflector for adjusting the coupling factor, one may also adjust theresonance frequency of the cavity ν_(cav). However, and as will be shownlater, the optimisation of the coupling factor need not be coincidentwith the tuning of ν_(cav) to equal the generated frequency. In apreferred embodiment, both the waveguide and the waveguide applicatorpreferably supports a TE₁₀ mode so that the condition of field matchingis fulfilled.

[0031] A waveguide applicator is in its simplest form a waveguideterminated by e.g. a short circuit wall, an iris or equivalent, which isadapted to hold a sample to which the microwaves are applied. Thus awaveguide applicator supports the same TE or TM mode as the waveguide ofwhich it is an end-part. Depending on the waveguide and the mode in thewaveguide, the applicator need not have the exact same cross-sectionaldimensions as the waveguide. Typically, the waveguide supports a TE₁₀mode wherein the electric field has no variations in the verticaldirection, hence, in this case only the horizontal dimension (the width)of the waveguide and the waveguide applicator needs to be at leastsubstantially equal. The geometrical constraints between the waveguideand the waveguide applicator for different designs will be obvious forthe person skilled in the art bearing the need for field matching inmind.

[0032] A single mode applicator is an applicator comprising anapplicator cavity adapted to support only a single resonant mode withinthe frequency spectrum of the applied radiation. Hence, a waveguideapplicator is also single mode applicator, and depending on the context,the waveguide applicator may also be denoted a single mode applicator,or simply an applicator.

[0033] In order to reach high a high field strength within theapplicator, it is preferable that the resonance frequency of the cavityis close to or substantially equal to the frequency corresponding to anamplitude maximum in the generated frequency spectrum. The resonanceconditions can be expressed either as a tuning of the reactive impedance(the capacitative and inductive reactance) of the applicator, or as anadaptation of the electrical length of the applicator to make it equalto λ/2, where λ is the wavelength of the applied radiation.

[0034] The electric length is a measure of the distance traversed byelectromagnetic radiation in a medium in time t, and is approximatelyequal to the corresponding distance electromagnetic radiation would havetraversed in vacuum in the same time t If e.g. a high permittivitymedium of length x is inserted in a radiation path, the electrical pathlength is increased by (n−1)x, where n is the refractive index of themedium.

[0035] According to the present invention, the deflector is formed by aclosed loop defining a plane. In this plane, the deflector has a width“a” and a height “b”. Also in this plane, the material forming the loophas a radial thickness “c”. The deflector has an axial thickness “h”along an axis normal to the plane of the deflector. The circumference ofan inner perimeter of the closed loop of the deflector determines theinherent resonance frequency ν_(defl) of the deflector, and by that thefrequency of maximal blocking, when it is placed with its planeperpendicular to the direction of power flow in a waveguide. Thedeflector may be rotated so as to have its plane perpendicular to (orits axis parallel with) the waveguide where it will efficiently reflectradiation having a frequency equal or close to ν_(defl) (blockingposition). Also, the deflector may be rotated to a position where itsplane is parallel with (or its axis is perpendicular to) the waveguide,where it will only reflect radiation comparable to that of a plate ofconducting material having the same profile (open position). In betweenthese positions, the deflector can be characterised by a complexreflection coefficient R(ν, α_(defl)) depending on the frequency andangle of rotation. Hence ν_(defl) and α_(defl) at least partly determinethe coupling of radiation between the waveguide and the waveguideapplicator. The phase of the complex reflection coefficient varies as afunction of the angle of rotation of the deflector. This may beinterpreted as that the position of the minimum of the standing(reflected) wave varies with the angle of rotation thereby introducing aphase delay or shift as the deflector is rotated.

[0036] As stated previously, the deflector forms a resonant cavity withthe waveguide applicator (with sample). As said above, the deflector mayaffect the electrical distance for at least part of the electromagneticwaves guided towards the applicator so as to virtually change theeffective length of the cavity. Since this effect depends in the angleof rotation of the deflector, the resonance frequency of the deflectormay be tuned by rotating the deflector.

[0037] Since the resonant frequency of the cavity may change when thepermittivity of the sample varies, the deflector action may compensatethis change, thus keeping the resonant frequency substantially constantand thereby provide a possibility to provide a high microwave heatingefficiency.

[0038] The complex reflection coefficient of the deflector, theresonance frequency ν_(cav) of the cavity, and the coupling of radiationbetween the waveguide and the cavity are closely related. Forillustrative purposes, the tuning of dimensions and the angle ofrotation of the deflector may be considered as a balance betweencoupling radiation to the cavity and keeping the coupled power in thecavity. If for example ν_(defl)=ν_(cav), the deflector in its blockingposition may form a very efficient “end mirror” for resonant radiationin the cavity, however, only very little radiation (having the rightfrequency ν_(cav)) may be coupled to the cavity. When the deflector isrotated towards its open position, more radiation may be coupled to thecavity, but on the other hand, the deflector may not form a veryefficient “end mirror”, and more power may be lost from the applicator.Thus at some position between blocking and open position, a maximum inthe power in the cavity may be expected. If on the other hand ν_(defl)is very different from ν_(cav), radiation having a frequency ν_(cav) mayefficiently be coupled to the cavity even when the deflector is in itsblocking position, but the deflector may not form a very efficient “endmirror”. Hence, and a maximum in the power in the cavity may be expectedat a ν_(defl) which is not equal to but neither too different fromν_(cav).

[0039] A proper choice of the axial thickness significantly larger thanthe radial thickness will provide a desirable location change of thephase of the reflected wave when the deflector is rotated. Preferably,the axial thickness of the deflector is in the interval [λ/₂₀;λ/₁₀],such as within the interval 3 to 25 mm in a 2450 MHz, TE₁₀ waveguidewith dimensions 86×43 mm (width x height). For waveguides with lowerheights, such as 25 mm, the axial thickness must be smaller; a suitabledimension has been found to be about 10 mm. Also in a preferredembodiment, the radial thickness of the deflector is between 0.1 mm and5 mm.

[0040] Preferably, the deflector is shaped like an ellipse having amajor principal axis of length a and a minor principal axis of length b.Alternatively, the deflector is shaped like a trapezium, such as arectangle having a width a and a height b. The choice of the detailedshape of the closed loop depends on the desired “leakage properties”,where an elliptical shape may give maximum blocking according to theprior art.

[0041] For a predetermined set of conditions such as sample volume,sample permittivity, position of the sample in the applicator, andcoupling of the guided waves between the waveguide and the applicator,the applicator may become anti-resonant. In this case, the resonancefrequency of the applicator and/or the coupling of the guided wavesbetween the waveguide and the applicator may be adjusted by comprising amember of a material having a relative permittivity larger than 5, suchas larger than 10, preferably larger than 25 positioned within theapplicator. In order to prepare relative permittivity of the material,it may comprise ceramic materials comprising one or more materialsselected from the group consisting of Al₂O₃, TiO₂ or XTiO₃, where X isany group II element such as Ca or Mg. The relative permittivity and/orthe shape and/or the size of said member might be chosen so as to makethe applicator resonant at said predetermined set of conditions.

[0042] Optionally, the apparatus may further comprise means foradjusting the position of the sample in the applicator in order toadjust the effect of the sample upon the resonance frequency of thecavity and/or the coupling of the guided waves between the waveguide andthe applicator. Preferably, the means for adjusting the position of thesample comprises means for adjusting a substantially vertical positionof said supporting means.

[0043] In order to reduce the amount of scattered waves towards thegenerator, the apparatus may further comprise a first circulator and afirst dummy load, wherein the first circulator is adapted to deflect atleast part of electromagnetic waves reflected from the applicatortowards the first dummy load. One or more power measuring means may bepositioned so as to measure the power of at least part of theelectromagnetic waves deflected by the first circulator. The one or morepower measuring means is preferably operationally connected to a firstmemory means for storing the measured power.

[0044] The generator may comprise a magnetron or a semiconductor basedgenerator and a semiconductor based amplifier. The semiconductor-basedamplifier preferably comprises one or more silicon-carbide powertransistors. Alternatively, the generator may comprise both a magnetronand a semiconductor based generator.

[0045] The sample is preferably held in a container which issubstantially hermetically closed and adapted to withstand pressure.

[0046] Also, it is often of interest to monitor the temperature of thesample during heating. For this purpose, the apparatus may comprise athermal radiation sensitive element adapted to determine a temperatureof the sample and positioned so as to receive thermal radiationemanating from the sample.

[0047] Both the high pressures and the high temperatures of the sampleimply a risk for the container to break and thereby leak sample in theapplicator. The breaking of the container can be such as an explosion orsimply a melting of the container. In order to protect the deflector andthe waveguide in case of breaking of the container, the apparatus maycomprise a screen for separating the deflector and the waveguide fromthe container so as. The screen is preferably substantially transparentto the electromagnetic waves guided towards the applicator, and maycomprise one or more of the materials selected from the group consistingof: PTFE (Teflon®) TPX, polypropene or polyphenylidenesulphide (PPS,Ryton®). Optionally, the applicator also comprises a drain for drainingsample from within the applicator. Preferably, the drain leads to areceptor for receiving the sample drained from the applicator.

[0048] The apparatus may be further automated by comprising means forplacing the sample within the applicator. If the sample is loaded intothe container outside the apparatus, the placing means is means forplacing the container at least partly within the applicator.

[0049] In order to allow for a larger variation in the power and/orfrequency of the generated waves, the apparatus may further comprise asecond generating means for generating electromagnetic waves. In thiscase the waveguide is adapted to guide at least part of theelectromagnetic waves generated by the first and second generating meansto the applicator. In order to allow parallel processing of samples, theapparatus may further comprise a second applicator for holding acontainer holding a second sample. In this case the waveguide is adaptedto guide at least part of the electromagnetic waves to the first andsecond applicator. The second applicator may also comprise all thefeatures described in relation to the applicator above. The combinationof two or more generators and two or more applicators allows for a largesystem wherein the generated power is dosed to each applicatorindividually.

[0050] The term microwave is intended to mean electromagnetic radiationin the frequency range 300 MHz-300 GHz. Preferably, the apparatus andmethods according to the invention are performed within the frequencyrange of 500 MHz-300 GHz, preferably within the frequency range 500MHz-30 GHz such as 500 MHz-10 GHz such as 2-30 GHz such as 300 MHz -4GHz such as 2-20 GHz such as 0,5-3 GHz or within the range 50-100 GHz.

[0051] In the present context, the term “apparatus” designates one orseveral pieces of equipment which, as a whole, comprise the parts, meansand elements that characterise the invention. Accordingly, the apparatusmay appear as a distributed system where individual parts or means arenot located in close physical proximity to each other. As an example ofthis architecture, the memory means may be physically located on e.g. apersonal computer (PC) while all the mechanical parts may appear as ajoint unit.

[0052] In a second aspect, the present invention provides a method forapplying the apparatus of the first aspect. Thus, according to thesecond aspect, the present invention provides a method for heating asample, said method comprising the steps of:

[0053] I. providing a heating apparatus according to the first aspect,and inserting the sample in the applicator,

[0054] II. generating electromagnetic radiation at a first output powerlevel,

[0055] Ill. rotating the deflector for adjusting the coupling factorbetween the waveguide and the resonant cavity.

[0056] When a heating process is initiated, the sample has a firsttemperature T₁. The method preferably further comprises the steps of:

[0057] heating the sample to obtain a second temperature T₂>T₁,

[0058] rotating the deflector for adjusting the coupling factor betweenthe waveguide and the resonant cavity in response to the variation inthe dielectric properties εsample of the sample.

[0059] The above steps may be repeated several times during a heatingprocess.

[0060] The present invention allows for designing and/or optimising of aheating process of a sample. Thus, the method according to the secondaspect may further comprise the steps of:

[0061] IV. performing the following steps one or more times:

[0062] positioning the deflector in a first position and measuring afirst power of electromagnetic radiation reflected from the waveguideapplicator, the reflected radiation corresponding to said first positionof the deflector,

[0063] rotating the deflector to a second position that is differentfrom the first position and measuring a second power of electromagneticradiation reflected from the waveguide applicator, the reflectedradiation corresponding to said second position of the deflector, and

[0064] V. determining a preferred position of the deflector based on theamount of power reflected from the waveguide applicator in at least thefirst and second position.

[0065] These measured powers are preferably inversely proportional tothe power absorbed in the sample at the first and second position of thedeflector. Preferably, this designing and/or optimising are onlyperformed once for each type of sample or reaction since the obtainedparameters can be saved for later use. Hence, the method may furthercomprise the steps of:

[0066] VI. providing a first storing means,

[0067] VII. storing information relating to the first position in thestoring means and storing the measured first power in relation thereto,and

[0068] VIII. storing information relating to the second position in thestoring means and storing the measured second power in relation thereto.

[0069] It will often be of interest to store measured powerscorresponding to a plurality of different positions, and the steps IV,VII, and VIII may be repeated as often as desired. The deflector anglesand the powers may be stored as a listing such as a table, in thestoring means. According to the second aspect, step V may compriseprocessing of the stored measured powers for determining the preferredposition of the deflector corresponding to a local or an absoluteminimum in the measured power, or to a predetermined ratio of themeasured power to the first output power level.

[0070] After the determination of a preferred position of the deflector,the method may further comprise the steps of positioning the deflectorin he preferred position in order to heat the sample. Optionally, themethod also comprises the step of, after having positioned the deflectorin the preferred position, generating electromagnetic radiation at asecond output power level which is larger than the first output powerlevel in order to heat the sample at a higher rate.

[0071] By comparing the stored measured powers with corresponding storedmeasured powers measured for a different second sample, it is possibleto determine a measure of the relative permittivity of a first samplerelative to the relative permittivity of the second sample.

[0072] Alternatively, by comparing the stored measured powers withcorresponding stored measured powers measured for a second sample ofknown chemical composition, it is possible to determine an indication ofthe chemical composition of the first sample relative to the chemicalcomposition of the second sample. If the first sample comprises at leastone reactant for performing a chemical reaction, the method may furthercomprise the steps of:

[0073] performing the chemical reaction with the at least one reactant,and

[0074] determining a degree of reaction for the chemical reaction usingthe indication of the chemical composition of the sample,

[0075] where the degree of reaction is a measure of the extent to whichthe reactants has reacted to form products in a chemical reaction.

BRIEF DESCRIPTION OF FIGURES

[0076]FIG. 1 is a cross sectional view of a first embodiment of theapparatus according to the present invention.

[0077]FIG. 2A illustrates electric and magnetic field lines in awaveguide according to the first embodiment of the present invention,FIG. 2B shows magnetic field lines and currents in the waveguide walls.

[0078]FIG. 3 is an illustration of a deflector according to the presentinvention.

[0079]FIG. 4 is a flow diagram describing procedural steps of a heatingprocess according to the present invention.

[0080]FIG. 5 is a diagram with a curve showing temperatures and timeintervals of a heating process according to the present invention.

[0081]FIG. 6 is a diagram with a curve showing a sketch of a typicalfingerprint of a sample according to the present invention.

[0082]FIG. 7 shows an illustration of a test rig used for experimentalverification of the properties of the apparatus according to the presentinvention.

[0083]FIG. 8 to 13 are various graphs showing experimental data obtainedusing the test rig of FIG. 7.

[0084]FIG. 14 shown a schematised drawing of an apparatus according tothe present invention used in a computer simulation used for theoreticalverification of the properties of the apparatus according to the presentinvention.

[0085]FIG. 15 to 20 are various graphs showing data obtained from thesimulation.

DETAILED DESCRIPTION OF FIGURES

[0086] In the following, a specific embodiment of an apparatus accordingto the invention is described and discussed in greater detail. Thepresent description provides a more detailed description of preferredfeatures of the invention, described in relation to the preferredembodiment. However, it will be understood, and will be realised by theperson skilled in the art, that the invention is not limited to thepresently discussed embodiment, and that each of the individual featuresdescribed in the present embodiment could be implemented in many otherways. Also, experiments as well as computer simulations verifying theperformance of the present invention is presented.

[0087] In a preferred embodiment, the present invention relates to amicrowave (MW) heating apparatus for heating a sample with an improvedefficiency. The improved efficiency is 15 achieved by applying a numberof features including:

[0088] a single mode waveguide applicator,

[0089] means for adjusting a resonance frequency of a resonant cavity inthe waveguide applicator in response to the variation in dielectricproperties of the sample during heating, in order to make the waveguideapplicator resonant and ensure a high field strength inside thewaveguide applicator, and

[0090] means for adjusting the coupling factor of MW radiation between awaveguide and the applicator in order to optimise the radiation coupledto the sample.

[0091] As previously mentioned, a single mode applicator is anapplicator comprising an applicator cavity, which is adapted to supportonly a single resonant mode within the frequency spectrum of the appliedradiation. In this case, the mode in the applicator is the normal modein its parallelepipedic shape, namely the TE₁₀₁ first rectangular mode.The normal mode is defined as the first propagating mode, which appearsin when the frequency of the generator is increased from 0 Hz. Thesample, which can have a substantial and varying permittivity, canmodify the details of this mode field pattern, but the quotient betweenthe sample and applicator volumes is still so low that the simple modepattern essentially remains. The resonance criteria for single modeapplicators are in general more critical than for multimode applicators,since the relative strength of the overlapping modes will“automatically” change in a well-designed multimode system when thesample changes, in order to retain a good coupling factor. However, thechange of the mode balance also means that the heating pattern in thesample is changed, which results in an inhomogeneous heating. Thepattern change is amplified by the fact that a multimode resonant cavitymust be much larger in size than a single normal mode applicator inorder to support the higher order modes, which means that the resonantfrequency bandwidths of each mode become smaller. The spatial energydistribution within a single mode applicator is more uniform having moreintense but fewer and remaining maxima than the spatial energydistribution of a multimode applicator. However, if the resonance andthe coupling can be controlled, and if the position of the sample ischosen properly, the field strength at the position of the sample can beconsiderably larger for a single mode applicator compared to multimodeapplicators. This is due to the fact that a multimode cavity must have amuch larger volume than a normal mode applicator. Thereby the power lossin the cavity walls becomes much higher.

[0092]FIG. 1 shows a cross sectional view of a preferred embodiment ofthe present invention. The apparatus has three arms in which a microwavegenerator 2, an applicator 4, and a dummy load 5 are placedrespectively, and a waveguide part 3, forming two of the arms, forguiding the radiation from the generator 2 to the applicator 4. Adeflector 26 is positioned in the waveguide 3 near the applicator 4.

[0093] The apparatus of the preferred embodiment further comprises acontroller 7 such as a computer, which can store and process measuredvalues and control the generator power. Optionally, the controller 7also monitors parameters such as temperature and time and controlsfunctions such as cooling and deflector position. The controller isoperationally connected to the generator 2 and to devices such as theIR-sensor 32, power measuring means 21 and 22, deflector driver 27,cooling means (not shown) and height adjustment means (not shown). Thecontroller has a user interface, allowing the user to set-up a specificheating process.

[0094] The microwave generator 2 can be a magnetron or a semiconductorbased generator. In the case of a magnetron, the magnetron 2 is mountedon the top of the waveguide 3 with its antenna 16 inserted into thewaveguide 3. For the power range 1 to 300 W, the magnetron is preferablycharacterised by control of the input power with an electromagneticsolenoid, which is used to regulate the magnetron output power bychanging the static magnetic field in it. The frequency band istypically centred at either 2450 MHz or 915 MHz. The temperature of themagnetron is preferably measured with a temperature sensor (not shown)and the magnetron is allowed to reach 90 ° C. as a maximum temperature,again in order to stabilise its operation.

[0095] In the case of a semiconductor-based generator (not shown), thegenerator may also be a semiconductor-based amplifier using e.g.silicon-carbide power transistors. Semiconductor based microwavegenerators and amplifiers provide a variety of advantages overconventional TWT's (Travelling Wave Tube), gyrotrons and magnetrons.Examples of these advantages are:

[0096] Easy control of frequency and output power

[0097] Small physical dimensions

[0098] No high voltage required, which improves safety and reliability

[0099] No warm-up time, therefore immediately availability andpossibility of fast variation in output power

[0100] No wear-out parts which significantly reduce cost maintenance andimprove apparatus up-time

[0101] Far higher MTBF and lower MTTR compared with TWT

[0102] Better gain curve flatness compared with TWT

[0103] Lower noise compared with TWT

[0104] The amplifying means preferably has a signal amplifier and apower amplifier. The signal amplifier is a semiconductor-based devicebeing adapted to amplify the signal from the signal generator. The poweramplifier is provided for further amplifying the signal from the signalamplifier, and is also a semiconductor-based device. The gain of thesignal and power amplifiers are adjustable and the operator or thecontrolling device can select the amplitude of the output by setting thegain of the power amplifier.

[0105] Since the frequency of the radiation generated by asemiconductor-based generator is variable, it offers furtherpossibilities in optimising the heating procedure in that the frequencycan be tuned to an absorption maximum of the sample.

[0106] Alternatively, the generator 2 is a combination of a magnetronand a semiconductor based generator (not shown), each operating indifferent power and/or frequency regimes. This constellation provides alarge degree of flexibility and power economy since each generatordevice may be used for the purpose at which it has its strengths.

[0107] The waveguide 3 in the preferred embodiment is a rectangularwaveguide. For a rectangular waveguide, the normal mode is a TE₁₀ mode(Transverse Electric Mode) using the following terminology in relationto FIG. 2: Coordinate x y z Waveguide dimension — b a Mode p n m

[0108] with TE_(mn) for waveguides and TE_(mnp) for a cavity. In FIG.2a, the magnetic field is shown as the elliptical dashed lines and theelectric field is shown as straight solid lines between the top and thebottom. The wall currents are displayed in FIG. 2b. In the firstembodiment, the waveguide has a rectangular cross section, however,other cross sectional shapes such as elliptical can be used.

[0109] Due to the current flow shown in FIG. 2b, there has to be goodcontact between the side walls and the horizontal parts of both thewaveguide and the applicator in order not to leak microwaves. In thefirst embodiment, a sealing material, consisting of a silicone rubberstrip with metal threads around forms this contact. The rubber strip ispositioned between the assembled parts.

[0110] It is of interest to measure the power applied to and reflectedfrom the applicator. In case where a magnetron is used as generator, itis also of interest to reduce radiation feedback to the generator, whichwill otherwise cause fluctuation of the operating power and frequency,and reduce the lifetime of the magnetron. For these purposes, thewaveguide contains a circulator 17 and two power measuring means 21 and22.

[0111] The circulator 17 comprises two magnets 8, two special ferrites19 and three stubs (metal posts) 20. The function of the circulator isto direct the electromagnetic radiation in certain directions dependingof its direction of propagation. In the present configuration, thecirculator is adapted to transmit radiation travelling from thegenerator 2 towards the applicator 4, but deflect radiation travelingthe opposite direction into a dummy load 5. Since the waveguide isessentially symmetric in two of the three arms, the magnets 18 and theferrites 19 should be placed along a symmetry axis of the circulator andtowards the dummy load 5.

[0112] The positions of the stubs 20 (one towards the generator 2, onetowards the dummy load 5 and one towards the applicator 4) should inprinciple be symmetrical and the stub close to the dummy load should beoptimised so that only −17 to −20 dB is reflected back to the generator.Since the current flows along the axis of the waveguide in the middle ofthe waveguide (see FIG. 2B), it is possible to leave open slots foradjusting the position of the stubs without microwave leakage. Thefunction of the stubs is to act as a phase compensating capacitance andincrease the efficiency of the circulator. Since the dummy load 5 cantransfer heat to the circulator 17, the temperature of the circulator ismeasured with a temperature sensor (not shown) close to the dummy loadarm. The maximum temperature allowed for the ferrites is 70 ° C.

[0113] The power sensor 21 is a common crystal detector placed so as tomeasure the power of the radiation reflected from the applicator anddeflected by the circulator into the dummy load arm. Since the dummyload is matched, there are no standing waves in this arm. This meansthat the signal as measured anywhere in that arm is proportional to onlythe reflected power by the sample to be heated.

[0114] Knowing the losses in the waveguide and the ratio of reflectedradiation deflected by the circulator, allows a determination of anestimate of the power P_(refl) reflected from the applicator. The powermeasuring means 22 is a powermeter placed so as to measure the power ofthe generated radiation travelling towards the applicator. However, itmay also be exposed to radiation reflected from the applicator since theefficiency of the circulator is no 100%. Knowing again the losses in thewaveguide and the ratio of reflected radiation transmitted by thecirculator, allows a determination of an estimate of the powerP_(reciev) received by the applicator.

[0115] By determining the powers P_(refl,0) and P_(reiev,0) with anempty sample container (where all power is reflected) at the given powerlevel, the power absorbed by the sample can be determined by:

P _(abs)=(P _(reciev) −P _(refl))−(P _(reciev,0) −P _(refl,0)).

[0116] Often, the main interest is in the relative power absorbed in thesample for different conditions such as deflector position ortemperature. For this purpose, the direct power measured by powermeter21 is sufficient for determining relative values.

[0117] The powers P_(refl) and P_(reciev) are preferably measured as afunction of the generated power in the generator, such as the current Isupplied to the generator at fixed voltage, or equivalently the voltageor any other parameter characterising the generated power. The generatedoutput power is normally a direct function of the magnetron anodecurrent, which can be measured by the controller 7. Optionally, thedeflector may be put in a predetermined position where the applicator isout of resonance (with empty container). Thereby no strong fields occurat the position of the container, and the reading of the true generatorpower becomes more accurate. Alternatively, an estimate for the powerreceived by the sample, P_(reciev), can be determined directly from theparameter characterising the generated power as described above, makingthe power measuring means 22 insufficient.

[0118] The power measuring procedure described above power absorbed inthe sample can be determined using a number of different ways withvarious positions of powermeters, e.g. inside the applicator. The keyfeature is to determine an at least approximate value of the powerabsorbed by the sample.

[0119] The power measuring means 21 and 22 are connected to a storagemeans and preferably also to processor means. Preferably they areconnected to the controller 7. Alternatively, the power measuring meansthemselves comprise both storing and processor means.

[0120] The dummy load preferably comprises a material, which absorbsmicrowaves very efficiently independent of the temperature of thematerial, such as silicon carbide. The energy is converted into heat,which is led away by a cooling block. The dummy load absorbs the powerdeflected by the circulator. The position of the dummy load should benominally at the bottom of the arm.

[0121] As can be seen in FIG. 1, the applicator 4 has a largercross-sectional height than the waveguide 3, however, thecross-sectional width is constant, the same for both the applicator andthe waveguide. Due to the full compatibility between the field patterns,the waveguide is a TE₁₀ waveguide meaning that the electric field has novariation in the vertical y-direction. Hence, the waveguide willfunction as a TE₁₀ waveguide independently of any height variation.However, a low height is favourable for the circulator function andsaves space, and a higher waveguide in the applicator section isdesirable since a higher load can then be used. The difference in theheight between the waveguide and the applicator is therefore of noinfluence, and the applicator is simply a terminated waveguide—hence theterm waveguide applicator. Such a waveguide applicator is obviouslysingle mode as the waveguide is—hence the term single mode applicator isalso used.

[0122] For other designs than the one illustrated in FIG. 1, a waveguideapplicator may have different constraints to its dimensions, which willbe evident for the person skilled in the art.

[0123] The applicator as illustrated in FIG. 1 preferably comprises asample container holder 24, a protection screen 28, a cooling mechanism(not shown), and an IR-sensor 32. The applicator can have a cylindricalopening 34 at the bottom to allow for container explosions to bedirected downwards to a removable receptor 35 which can be emptied.

[0124] The container holder 24 is a tube of PEEK (poly-eter-eter-keton)for a sample container 6 and protects the container 6 from scratchesthat could be caused by the applicator when the container is put inplace. Scratches on a glass container decrease the maximum pressureallowed before explosion of the container occurs.

[0125] Since there is only one resonant TE₁₀₁ mode in the applicator,the position of the sample volume is an important parameter for assuringa good interaction between the sample and the microwaves and therebyoptimising the absorbed power in the sample. This is because theelectromagnetic field strength of the resonant mode is stronglydependent on the position. The horizontal position of the sample isdefined by the position of the container holder 24 and is determinedduring construction of the apparatus, whereas the vertical position isdetermined by the volume of the sample 8. Therefore the applicatorpreferably comprises means for adjusting the vertical position of thesample 8 (not shown in FIG. 1). Such means can be an adjustablesupporting plate on which the container 6 rests. Alternatively, theupper rim or the lid of the container 6 rests on the height adjustableupper neck of the container holder 24.

[0126] Since the samples can become up to 250° C., the material chosenfor the tubing has to be able to withstand at least 250° C. without anymechanical or chemical charges. A typical sample container is a glassvial, dimensioned specifically to withstand pressure without unduedeformation. Preferably the container is at least substantiallyhermetically closed in order to heat samples above the boilingtemperature at atmospheric pressure.

[0127] The protection screen 28 protects the deflector, the waveguideand part of the interior of the applicator in case of explosion of acontainer. It is made of an at least substantialy microwave penetrablematerial such as PTFE (Teflon®), TPX, polypropene orpolyphenylidenesulphide (PPS, Ryton®). The dielectric properties of thescreen affect the electrical length of the applicator, and the optimaldimension in this first embodiment is approximately 8 mm thickness.

[0128] The sample is preferably cooled by cooling the container withpressurised air, which is applied via several outlet holes next to thecontainer at the top of the sample holder (not shown). As an example,the sample is cooled for ten seconds after it has reached 40° C. due tothe time lag in the temperature measurements, which will be discussedbelow.

[0129] The IR-sensor 32 is placed so that it monitors the lower part ofthe container trough an opening 33 in the applicator wall. Preferably,the IR-sensor 32 is unsusceptible to microwaves and need not beprotected. However, in order to avoid leakage trough the opening 33, theopening must be secured either by a chimney, a metal grid or a sealedcasing for the IR-sensor 32. The IR-sensor should preferably monitor apart of the glass container that is in direct contact with the sample,else large measurement errors may occur. Since the IR-sensor measuresthe temperature on the surface of the glass there will be a differencebetween the real temperature (in the sample) and the measuredtemperature resulting in a time lag of up to 5 seconds for the realtemperature measurement. The IR-sensor is sensitive to condensed liquidson its surface since it then will measure the temperature of thecontamination and hence it is very important to keep it clean (e.g.after explosions).

[0130] The IR-sensor 32 is connected to a storage means and preferablyalso to processor means. Preferably it is connected to the controller 7.

[0131] Providing a deflector 26 in the waveguide 3 near the applicator 4forms a cavity between the termination of the applicator or the sampleand the deflector. Microwaves transmitted by the deflector and having afrequency equal or close to the resonance frequency of the cavity canform standing waves in the cavity. Hence, when mentioning a resonancefrequency of the applicator, a resonance frequency of the cavity formedby the applicator, sample, and deflector is actually meant.

[0132] Since the volume, real permittivity and loss factor of the samplevaries for different temperatures and different samples, the insertionand heating of a sample in the applicator changes the resonant frequencyof the cavity. The deflector 26 is adapted to compensate for thesedifferences in dielectric parameters of samples. The deflector 26 asshown in FIG. 3 is formed by a closed loop of a conductive material, andthe size and shape are adapted to the size and shape of the waveguideand the applicator. The deflector can be rotated around an axis ofrotation intersecting the loop as shown in FIG. 1. The axis of rotationneed not be a symmetry axis of the deflector. The varying inductive andcapacitative behaviour of the deflector provides adjustment of both itselectrical length, and hence the resonance frequency of the applicator,and the coupling factor between the applicator and the waveguide byimpedance matching. The loop of the deflector defines a plane, and thethickness of the loop perimeter along an axis normal to this plane isused for modifying the electrical length of the applicator. Thecircumference of the inner perimeter of the loop determines a resonancefrequency of the deflector.

[0133] The deflector design is adapted to simultaneously change theelectrical position of the deflector (the electrical distance from theapplicator end-wall or the sample to the deflector) in the TEM₁₀applicator/waveguide and its scattering/deflection properties when it isrotated. Thus, the rotation of the deflector describes a single curvedescribing “wave choking” (deflection is decibels) as a function of theelectrical position of the deflector. This curve has to beexperimentally optimised for the desired range of samples andtemperatures during design and construction. The deflective propertiesare adjusted by changing the size and shape of the elliptical loop.Typically, a remaining transmission should occur in the most blockingposition intended for samples with very small absorption capability,since this will result in a lower position sensitivity of the deflectorfor such samples. The axial thickness of the loop determines how muchthe electrical position changes when it is rotated; this is what resultsin its property of changing the resonant frequency of the cavity.

[0134] The conductive material of the deflector is preferably aluminium,which should have a high quality since the current density induced inthe deflector is high enough to make normal aluminium corrode. Therotation of the deflector is controlled with a step motor 27. Thedeflector has a twofold symmetry axis and hence the interesting anglesare 180 degrees. Optionally, the deflector may also be translated inorder to adjust the length of the applicator. Alternatively, the shapeof the deflector can be adjusted, or its axis of rotation can bedisplaced.

[0135] The power sensing in the dummy load arm provides an unambiguousindication of the relative (with respect to other deflector positions)applicator efficiency. Hence the power measurement P_(refl) is used forcontrolling the deflector. The deflector can be swept 180 degrees fordetermining the angle corresponding to maximum absorption (=minimumreflection) of power in the sample.

[0136] Alternatively, the deflector is non-conducting, but made from ahigh permittivity material (the word deflector is still used even thoughthe deflective properties are more pronounced in case of a conductivematerial). Adjustment of such a deflector varies the electrical lengthof the applicator and the capacitative reactance allowing for impedancematching between the applicator and the waveguide.

[0137] The deflector can only reduce the volume dependence and noteliminate it completely. At some volume(s), anti-resonance conditions ofthe applicator with sample may not be compensated for by the deflector,thus there will be a local minimum in the efficiency. In the apparatusaccording to the invention, such anti-resonance conditions occur atsample volumes of about 3 ml. However, such anti-resonances may becompensated for by including a member adapted to become resonant only atthe specific volume of the antiresonance. This member can be a materialof which the size, shape, relative permittivity and position within theapplicator is adjusted so as to make the applicator resonant at theconditions where the anti-resonance occurs. These conditions can bedetermined by the sample volume, as mentioned, but can also be at leastpartly determined by the coupling factor, the resonance frequency of theapplicator, the chemical composition or the temperature of the sample,the container or other parameters. Preferably, the material of themember has a high relative permittivity and is preferably a ceramicmaterial such as a material comprising Al₂O₃, TiO₂ or XTiO₃ where X is agroup II element.

[0138] Measurements have now been made on the behavior of a deflector ofthe kind used in the Lynx system (having an axial length of about 9 mm)and of a similar deflector with only 3 mm axial length. The measurementswere made with a precision waveguide system consisting of acoaxial-to-waveguide transition, an intermediate waveguide section(TE₁₀, with the same dimensions as in the Lynx system: 25×86 mm), andfinally another waveguide-to-coaxial transition, loaded by a perfectmatching resistor. The measurements were made at three frequencies, toascertain that any deviating inherent resonant frequency of thedeflector was considered.

[0139] The deflector virtually changes the active applicator length inorder to match a standing wave maximum to a heated sample with differentdielectric properties. The deflector is formed like an elliptic ringwith a specified thickness. This thickness is of crucial importance fora proper deflector function. The reflection coefficient and phase factorhave been determined using a network analyser and a specially designedtest rig.

[0140] A specially designed test rig was constructed to exclusivelystudy the effects of the deflector on the reflection coefficient andphase behaviour of irradiated microwaves. The test rig is schematicallydepicted in FIG. 7A. The rig is divided in three parts, where part 62 aTE₁₀—wave guide terminated in a 50 Ω load 70, part 64 a TE₁₀—wave guidedeflector section, and part 66 is a TE₁₀—wave guide connected to anetwork analyser 70. FIG. 7B shows a cross section of all deflectorparts having dimensions a=86 mm and b=25 mm. A deflector 26 is placed inthe middle of the deflector section 64 and at a height of 12 mm. Thedeflector 26 can be rotated 180° around its axis. The complex reflectioncoefficient was measured experimentally at 2440, 2455 and 2470 MHz withan HP8719A network analyser for different deflector angles.

[0141] The deflector 26 used in the test is a three-dimensional ellipticring made of aluminium similar to the deflector shown in FIG. 3. Twodifferent deflectors have been tested here: one with a thickness of 8.90mm and one with a thickness of 3.10 mm. The closed loop of the deflectordefines a deflector plane which is the plane of the paper in FIG. 3A.Also, the closed loop defines an axis shown in FIG. 3B which is normalto the deflector plane. The dimensions of the detector used in the testare summarised in Table 1. TABLE 1 Height a [mm] Width b [mm] Axialthickness h [mm] 17 68 8.90/3.10

[0142] The experimental data obtained in the test rig can only be usedqualitatively if the zero-phase at the deflector position is not known.This phase can be determined and compensated using the following method.

[0143] Part 62 of the test rig is removed from part 64 and replaced witha short circuit wall of aluminium. No deflector is mounted in thedeflector section. The amplitude and phase of radiation reflected fromthe short circuit wall was measured at 2440, 2455 and 2470 MHz. The dataare presented in Table 2. TABLE 2 Reflection coefficient data for theshort-circuit measurements Frequency (MHz) Amplitude (mV) Phase (°) 2440980 −169.5 2455 977 177.3 2470 981 166.4

[0144] These values compare to the reflection coefficient at the shortcircuit wall. The phase factor at the deflector position can becalculated by first measuring the distance L from the short circuit wallto the deflector position. The L-distance is determined to 58.43 mm. Thephase is turned counter clockwise when you move towards the generator.The phase turns 180° for every λ_(g)/2, i.e. half of the guidewavelength. Therefore the shift in phase when moving from theshort-circuit wall to the deflector position can be determined accordingto: $\begin{matrix}{{\Delta\phi} = {{\frac{2L}{\lambda_{g}} \cdot 180}{^\circ}}} & (1)\end{matrix}$

[0145] The wave-guide wavelength λ_(g) for the different frequencies iscalculated using the formula: $\begin{matrix}{\lambda_{g} = \frac{\lambda_{o}}{\sqrt{1 - \left( \frac{f_{c}}{f_{0}} \right)^{2}}}} & (2)\end{matrix}$

[0146] Here, λ₀ is the vacuum wavelength (=c₀/f₀ where c₀ is thevelocity of electromagnetic wave in vacuum), f_(c) is the cut-offrequency of the wave-guide and f₀ is the excitation frequency. Thecut-of frequency is given by the expression: $\begin{matrix}{\left( {f_{c}/f_{0}} \right)^{2} = {\left( \frac{\lambda_{0}}{2} \right)^{2}\left\lbrack {\left( \frac{m}{a} \right)^{2} + \left( \frac{n}{b} \right)^{2}} \right\rbrack}} & (3)\end{matrix}$

[0147] Here, (m,n) are the mode indexes (1,0 in our case) and a and bare the guide width and height respectively. The phase at the deflectorposition can then finally be calculated using the formula:

φ_(deflectorposition)×φ_(shortcut)+Δφ  (4)

[0148] The phase at the deflector position and other data are used forthe calculation are collected in Table 3. TABLE 3 Experimental andderived data for the zero phase calculations f₀ (MHz) λ₀ (m) λ_(g) /2(m)φ_(sc) (°) Δφ(°) φ_(dp) (°) 2440 0.12287 0.0878 −169.5 119.788 −49.7122455 0.12211 0.0867 177.29 121.308 −61.402 2470 0.12137 0.0857 164.4122.795 −72.805

[0149] The complex reflection coefficient (i.e. both the amplitude andphase) was measured for different angles in the interval 0-300° at thethree frequencies given in Table 3 above using the two different axialthickness given in Table 1. The results are presented in the nextsection.

[0150] The reflection coefficient amplitude for the 8.90 mm deflector isshown in FIG. 8 for different deflector angles and the three excitationfrequencies. The amplitude reaches above 800 mV in the interval 50-100°and 210-270° for all frequencies. The amplitude drops to just 150-200 mVin the interval 150-180° . This is in good agreement with the expectedresults illustrated in FIG. 6. The corresponding phase factor vs.deflector angle is depicted in FIG. 9 for the 8.90 mm deflector. Thephase is constant around 80° for a deflector angle in the interval50-100° and 210-270° for all frequencies, which coincide with theinterval for the amplitude maximum. FIG. 8. FIG. 10 shows both theamplitude and the phase of the reflection coefficient in a polar diagramwith the deflector angle as parameter at 2445 MHz (extracts from FIGS. 8and 9).

[0151] The reflection coefficient amplitude for the 3.10 mm deflectorthickness is presented in FIG. 11 for different deflector angles andthree different frequencies. The amplitude behaviour is similar to thevalues for the 8.90 mm deflector in FIG. 8. The phase factor for the3.10 mm deflector is shown in FIG. 12 for different deflector angles andfor the frequencies 2440, 2455 and 2470 MHz. The phase factor reaches avalue of 60° for the 2440 MHz in the interval 50-125° and 200-270°. Thismaximum in phase coincides with the maximum of reflection coefficientamplitude for 2440 MHz. However, the phase factors at 2455 and 2470 MHzare shifted to a minimum value of −60° in the same angle interval as the2440 MHz-curve. The thicker deflector is therefore capable of showing ahigh reflection coefficient amplitude and constant phase close to 90° inthe frequency band 2440-2470 MHz. The thinner deflector shows highreflection coefficient amplitude, but shows a lower absolute value ofthe phase factor and it changes sign in the frequency band. FIG. 13shows both the amplitude and the phase of the reflection coefficient ina polar diagram with the deflector angle as parameter at 2455 MHz(extracts from FIGS. 11 and 12).

[0152] In summary, the 8.90 mm deflector shows both a high reflectioncoefficient of 800 mV and a positive phase factor close to 90° for thethree frequencies 2440, 2455 and 2470 MHz. The 3.10 mm deflector showshigh reflection coefficient amplitude for the three frequencies, similarto the 8.90 mm deflector. The phase factor for the 3.10 mm deflector isalso lower and changes sign in the frequency band.

[0153] The most important feature to consider is how the phase of themismatch varies with deflector angle. The second most important featureis how the absolute value of the mismatch (i.e. reflection back by thedeflector) varies with deflector angle. A third, more practical feature,is how sensitive the deflector angle is with regard to changes of thetwo previous features, i.e. if the system becomes mechanically sensitivebecause of very rapid variation of data for small angle changes. Thephase curve for the 8.9 mm (normal) deflector shows that the phase movestowards the generator when it is turned towards a blocking position,i.e. deflector angle equal to 90° or 270°=with the axis along thewaveguide. Of course, since the deflector is passive and symmetrical itis also reciprocal, which means that also the phase on the “shadow side”(i.e. in the cavity) changes so that the resonant frequency mustincrease when the deflector is moved towards 90°. This behavior isdesirable. The phase curve of the 3 mm deflector behaves quitedifferently, the phase change is not in the desirable way.

[0154] Another important feature of the deflector is its blockingcapability in the blocking position. Even if it is possible to achievean extremely efficient blocking (so that perhaps less than 1‰ leaksthrough), this is not practical in heating systems, since too high fieldstrengths may then be achieved without a load or with a non-absorbingload. Actually, the former situation may cause heating until melting ofthe glass container. Hence, the deflector used is preferablyintentionally de-tuned to avoid that problem. This is evidenced by theblocking data. The de-tuning can be made either by detuning the inherentresonant frequency of the deflector, or by deforming it in such a waythat it leaks. In the preferred embodiment, the latter was chosen, bychoosing a non-optimal ellipticity. This choice further contributes tothe favorable phase variation with deflector angle.

[0155] In the experiments described in the previous sections, it was notpossible to vary the frequency continuously in order to find theresonance frequencies for the cavity for different detector angles andthereby directly show the change in the resonance frequency widthvarying deflector angles. However, despite the low resolution in thedeflector angle, different reflected amplitude minima for the threefrequencies is implied by the asymmetry of the minima between 150°-180°in FIG. 8. Clearly, the resonance frequency increases for increasingdeflector angle between 150°-180°.

[0156] The effect of the deflector in the waveguide in front of thewaveguide applicator has been modeled using the QWED s.c. (Poland) QW3Dsoftware. A complete model with both a rotatable deflector and arealistic load in the cavity have been used and resonant frequencies andcoupling factors as function of the deflector angle, with the loadpermittivity as parameter, have been obtained. The modeling softwareeditor image is shown in FIG. 14 with the waveguide 3, the deflector 26,the waveguide applicator 4, and a simulated load 61. The dimension ofthe waveguide, the deflector, and the waveguide applicator are similarto those used in the experiment described in relation to FIG. 7. Anumber of scenarios have been modeled where the reflection coefficientis calculated as a function of the frequency for a given deflector angleand load. Hence, for each scenario, the resonance amplitude can be readdirectly. The scenarios covers two axial thickness' of the deflector, 3mm and 10 mm and a number of different loads is used in the simulation,load #3, #4, and #5.

[0157] The graph in FIG. 15 shows the reflection coefficient as afunction of the frequency for the deflector at 90° which is the blockingposition (axis of the deflector parallel to the waveguide) and no load.As can be seen, the reflection factor at the resonant frequency of thedeflector 2435 MHz is 0.9999, meaning that only 1 minus this squared(i.e. 0.2‰) leaks through. The modeled deflector is more blocking thanthe real one due to software limitations, which necessitates drawing ofa perfect elliptical geometry of the deflector in order to allow quickand simple rotation in the scenarios.

[0158]FIG. 16 shows the amplitude of the reflection coefficient width adeflector angle of 60° using the 10 mm deflector and load #3. The wavyblack curve is obtained by the software after about 38000 iterations.However, the curve is by no means the stationary solution, so a specialoptional so-called Prony module was also used. It basically curve-fitsthe black curve to a number of Lorentzians, by a method far moreadvanced than inverse Fourier transformations. The resulting gray curveis clearly seen to be quite stabilized with a resonance at 2417 MHz. Theamplitude of the reflection coeficient at resonance is 0.45, but about0.9 at 2450 MHz. The deflector angle is thus not optimal.

[0159] By increasing the deflector angle to 65° (still 10 mm deflectorand load #3) one obtains the polar diagram shown in FIG. 17 which shownboth the amplitude and phase of the reflection coefficient as a functionof the frequency. Again, the black curve is obtained after a largenumber of iterations and the gray after having applied the Prony module.The system is now undercoupled (the curve in polar co-ordinates does notinclude the origo). Actually, the typical difficulty with any systemmatching device for low-loss loads is to avoid overcoupling;undercoupling means that the “choking” of the wave is stronger than 15optimum. The resonant frequency (lowest amplitude) is now up from 2417to 2428 MHz and amplitude of the reflection coefficient at resonance is0.27.

[0160] Using load #5 and the 10 mm deflector, the systems resonantfrequency is 2454 MHz and located at a deflector angle of 80°, which isalso the optimum for lowest amplitude of the reflection coefficient,0.37 overcoupled. This is shown in FIG. 18. The overcoupling compares toa coupling factor exceeding 1, which will sometimes happen especiallywith loads having a high absorption. When the angle of rotation iscontinuously changed in an overcoupled scenario, the coupling factorwill decrease continuously and must be 1 at some frequency for someangle of rotation. One should imagine a continuous transition from thepolar diagram of FIG. 18 to that of FIG. 17, the curve must intersectorigo at some angle and some frequency. This frequency needs not be theresonance frequency of the cavity and hence good matching (couplingfactor˜1) may be obtained at overcoupled nonresonant conditions.

[0161] Now, using the 3 mm deflector and load #4, the graphs shown inFIG. 19 and 20 was obtained. These graphs indicate that efficientresonant conditions are hard to achieve width the 3 mm deflector. Sinceboth cases are strongly overcoupled, it becomes possible to use a stillhigher deflector angle, about 88°. However, it becomes exceedinglysensitive to adjust. In an effort to establish good resonant conditions,the cavity size was changed several times by moving the position (axisof rotation) of the deflector. Thus, in the simulations used to obtainFIG. 19 and 20, the cavity is 12 mm shorter than that used with allmodeling with the 10 mm deflector.

[0162] The shown graphs are just a representative selection of theobtained results. The following conclusions summarize the results of themodeling:

[0163] 1. The 10 mm deflector provides desirable characteristics of thephase of the mismatch it is causing, by the standing wave phase movingclearly away from the deflector when it is rotated in the blockingdirection. This property is favorable for waveguide cavities to which itis coupled, these cavities operating for heating of variable loads(between loads and loads being heated).

[0164] 2. The angles of rotation needed to achieve the desired actionusing the 10 mm deflector are not as sensitive as for the 3 mmdeflector, and will thus provide a possibility for smooth regulation.

[0165] 3. There is, due to the very efficient blocking capability ofaxially long/thick deflectors, a possibility to “de-optimize” them intwo ways: by detuning of their inherent resonance (this is primarily bychanging the peripheral length), and by deforming them (this is bychanging the major/minor axis relationship, or using another curve formthat the elliptical) for increased leakage without a strong phasechange. The combination of these two possibilities and changing theaxial length provides a multitude of options to modify the phase andreflection factor (i.e. the polar curve shape) as function of thedeflector angle.

[0166] 4. The deflector with a long length allows higher power handling.

[0167] In another preferred embodiment, the apparatus is adapted toperform plurality of heating processes in plurality single modeapplicators simultaneously. In this second embodiment, the apparatuscomprises one or more generator, two or more single mode applicators,and a waveguide adapted to guide radiation from the one or moregenerators to the two or more applicators. The waveguide is furtheradapted to distribute the guided radiation between the applicators,preferably by comprising components such as couplers, dividers,splitters, combiners and circulators.

[0168] Each of the single mode applicators preferably comprises the samefeatures as the applicator of the first embodiment, 4 in FIG. 1. Theapparatus also comprises a controller similar to the controller 7 of thefirst embodiment, further adapted to administrate the heating processesof all samples in the two or more applicators. The apparatus accordingto the invention is suited for performing chemical reactions such asorganic synthesis, where a fast heating of a reaction mixture to apredetermined temperature is crucial to the purity of the final product.The reaction mixture may comprise one or more reagents such as organiccompounds and optionally a catalyst. Often, the reaction mixturesuccessively undergoes several reaction steps as in the processillustrated in FIG. 5, each for a given time at a specific temperature.Since different reaction mechanisms may dominate at differenttemperatures, the purity of each reactive step depends upon a highheating rate between the desired temperatures. The heating rate,^(dT)/_(dt), is the temperature rise in the sample per time unit,typically measured in °C./sec, and corresponds to the gradient of thecurve segment 41 in FIG. 5.

[0169] A heating procedure of a preferred embodiment is described inrelation to the flow diagram in FIG. 4. Initially, in step 50, the userconfigures the heating process through the controller interface. Thisprocedure comprises specifying the temperature T₁ the sample shouldreach and the time interval t₁ the sample should be kept at the constanttemperature T₁. If the heating procedure is a series of reaction steps,this leads to a sequence T₁, t₁; . . . . ; T_(i), t_(i); T_(n), t_(n) ofreaction temperatures T_(i) and corresponding time intervals t_(i)corresponding to the heating/cooling process shown in FIG. 5.Temperatures T_(i) are hereafter referred to as target temperatures andtime intervals t_(i) as target times.

[0170] Steps 51 through 54 in FIG. 4 describe a calibration procedurefor determining some relevant properties of the applicator 4 with thespecific sample 8. These relevant properties are obtained by recordingthe absorbed power in the sample during an operation cycle in deflectormotion (e.g. 180° rotation or rotation and translation). Optionally,only relevant intervals in deflector motion are recorded. The recordedtrace of absorbed power versus deflector position is referred to as thefingerprint of the sample, and is specific to several parameters such asthe:

[0171] 1. applicator design,

[0172] 2. container design and material,

[0173] 3. sample volume,

[0174] 4. irradiated power,

[0175] 5. centre frequency and frequency bandwidth of the radiation,

[0176] 6. temperature of the sample (=container+sample), hence the“heating history” and thereby the heating rate,

[0177] 7. chemical composition of the sample, hence its permittivity,degree of reaction etc. The parameters 1 through 5 can be held constantand are not related directly to the sample. Parameters 6 and 7 are theparameters of interest and hold information relating to the specificsample.

[0178] A rough sketch of a typical fingerprint for the rotatabledeflector 26 is shown in FIG. 6, where the reflected power isillustrated as a function of deflector angles from 0 to 180°. It is seenthat the fingerprint has a symmetry corresponding to the symmetry of thediagonal positions of the deflector, i.e. an angle of 45 and 135degrees. However, the two local minima a and b might have differentshape and depth due to the asymmetric shape of the applicator. Byprocessing the fingerprint, the deflector position corresponding tomaximum absorption (=minimum reflection) can be determined.

[0179] In step 51 of FIG. 4, the deflector 26 is set to an initialposition, which is preferably a position where neither maximum norminima typically occur. The reason for this being that it is desirableto minimise the power absorbed in the sample during calibration; if thedeflector starts near a typical maximum position, the sample will beexposed to undue power during the generator rise time. If the deflectorstarts near a typical absorption minimum, it will take unnecessary longtime to determine the absorption maximum with a correspondingly largeamount of absorbed power. A temperature rise in the sample duringcalibration is illustrated by the curve segment 40 in FIG. 5, but may betotally negligible.

[0180] The microwave generator 2 is started in step 52. The generator ispreferably set to an output power level of 10-20 W during calibration.If the generator is a magnetron, there might be a minimum output powerlevel for stable operation, this mininum level should be chosen if it islarger than 10-20 W. Semiconductor based generators have stableoperation at very low output power levels. In the alternativeconfiguration where the apparatus has a combination of a magnetron and asemiconductor based generator, the semiconductor-based generator ischosen in this low output power regime.

[0181] In step 53, the deflector is moved (continuously or stepwise)through a duty cycle such as a 180° rotation, and the reflected power ismeasured and stored for each angle to obtain a fingerprint. Optionally,the motion only covers a selected interval of interest in order tominimise the time spent and thereby the absorbed power. The output powerlevel (and frequency for semiconductor based generators) and thetemperature of the sample are preferably stored in relation to thefingerprint.

[0182] After the fingerprint has been recorded, the deflector positioncorresponding to absolute minimum in reflected power is determined instep 54, and the deflector is moved to this position. The apparatus isnow ready to start a fast, efficient heating of the sample.

[0183] Steps 55 through 58 in FIG. 4 is a feedback loop which heats orcools the sample to the target temperature and stabilises thetemperature around the target temperature, corresponding to the curvesegments 41 and 42 in FIG. 5. The generator output power or the coolingby pressurised air is adjusted in step 55 depending of the present setof target values T_(i), t_(i) and the present temperature T. In thebeginning of a heating procedure, the generator is preferably adjustedto the maximum output power level in order to achieve as large a heatingrate as possible. If the sample is to be cooled, it is cooled by coolinghe container with pressurised air (high cooling rate). Alternatively itis simply left to cool down by itself (low cooling rate).

[0184] Step 56 is the process that takes place, that is heating orcooling. When two or more starting materials reacts chemically they aresubject to changes in their physical and chemical properties, such aschanges in the dielectric properties. The energy transferred into thereacting materials is dependent of the dielectric properties of thestarting and formed materials during the chemical reaction. Thedielectric properties will therefore vary during the heating processresulting in a varying heating rate at different temperatures, asillustrated by curve segments 41 and 44 in FIG. 5. Therefore, it may beof interest to optimise the power absorption at temperatures betweeninitial and target temperature by a running adjustment of the deflectorin parallel with the heating. Hence the controller can optionally repeatstep 53 and 54 of the calibration procedure at predetermined intervals.Step 53 and 54 records the fingerprint determines the deflector anglecorresponding to absorption maximum and sets the deflector at thedetermined angle. This off cause implies a short period of reducedabsorption, but results in enhanced absorption.

[0185] The controller monitors the temperature T, and step 57 of FIG. 4is a check of whether the temperature T is larger or smaller than thetarget temperature T_(i). If the generator is on, the procedure is aheating procedure, and if T<T_(i) (=No if generator on), then theprocess 56 continues until next check (e.g. once every second). IfT>T_(i) (=Yes if generator on), a start time t₀ is set for calculatingthe time interval t_(i) (only if no current t₀ is set) and the procedureproceeds to step 58. If the generator is off, the procedure is a coolingprocedure and if T>T_(i) (=No if generator off), then the process 56continues until next check. If T≦T_(i) (=Yes if generator off), a starttime t₀ is set for calculating the time interval t_(i) (only if nocurrent t₀ is set) and the procedure proceeds to step 58.

[0186] Optionally, the controller can stop or step-down theheating/cooling when the temperature is within a certain interval of thetarget temperature in order to minimise or avoid target temperatureovershoot.

[0187] Step 58 checks whether t−t₀>t_(i), that is if the time intervalt_(i) has expired since the sample temperature T_(i) was reached. Ift−t₀<t_(i) (58=No) the procedure loops back to step 55 where thegenerator output power or the cooling is adjusted in response to thereading of step 57. The loop 55 to 58 is repeated until the target timehas elapsed and the procedure proceeds to step 59 (58=Yes).

[0188] In step 59 it is determined whether all steps in the processsequence defined in step 50 have been performed. If not (59=No), step 55through 58 is repeated with the new set of target values T_(i), t_(i).If all steps have been performed (59=Yes), all devices are turned off instep 60 and the sample can be removed from the applicator.

[0189] It is stressed that the procedure outlined above is a procedureaccording to a preferred embodiment. One or more steps may be changed,removed or added without changing the concept of the invention which isto execute a heating/cooling process such as the process illustrated inFIG. 5.

[0190] In a further embodiment, the controller may comprise, or haveaccess to, a database of fingerprints and heating rates at differenttemperatures and volumes for a number of solvents. When initialising theheating procedure in step 50, the user can further specify the volumeand the solvent so that the controller can find the relevant informationin the database. With this information the controller can optimise theheating procedure in one or more of the following steps:

[0191] Adjust the deflector to give optimal power absorption in thesample without initial calibration, this would remove step 51 through54.

[0192] Running adjustment of the deflector in parallel with the heatingin order to ensure optimal power absorption at temperatures betweeninitial and target temperature. Adjustment of deflector in e.g. step 57,but without performing any calibration since the optimal deflector angleat the current temperature is determined from the fingerprint in thedatabase.

[0193] A faster adaptation of the appropriate power or coupling factorduring a constant temperature feedback loop 55 to 58, e.g. throughintelligent guesswork. Since the volume, the solvent and the targettemperature is known the radiation losses can be calculated and thepower absorption can be adjusted thereto by adjusting the deflectorangle in correspondence with the fingerprint at target temperature.

[0194] Running determination of an indication of the chemicalcomposition of the sample by comparing the fingerprint with afingerprint of a known composition. This procedure may be advantageousin case of chemical reactions in the sample, since the degree ofreaction for the chemical reaction can be monitored by comparing thefingerprint with a fingerprint of the chemical composition of a samplewith the desired degree of reaction.

[0195] Database might be used to extract data resulting in a scalingfunction for each specific reaction:

[0196] S(T,P )=the absorbed power per volume unit [W/L] at a giventemperature T and given power density P,

[0197] where the power density P is the field strength at the positionof the sample (ideally constant through the sample). S can be used toderive heating procedures for other apparatuses with other samplevolumes, since it specifies the absorbed power and the heating ratedS/dT|P,T at given conditions, T and P, in said other apparatuses.

1. A heating apparatus comprising: generating means for generatingelectromagnetic radiation at a wavelength λ, a waveguide for guiding thegenerated electromagnetic radiation to a waveguide applicator forholding a sample to be heated, the sample having dielectric propertiesε_(sample) which varies as a function of a temperature of the sample,the waveguide and the waveguide applicator supporting a singletransverse mode, a deflector formed by a closed loop defining a plane,said deflector having an inherent resonance frequency ν_(defl) and athickness in the interval [λ/₃₀; λ/₅] in a direction normal to saidplane, the deflector being rotatable around an axis being at leastsubstantially parallel to said plane, the deflector being positioned inthe waveguide so as to form a resonant cavity with the sample and thewaveguide applicator, said cavity having at least one resonancefrequency ν_(cav) being dependent upon at least ε_(sample), ν_(defl),and an angle of rotation of the deflector, α_(defl).
 2. An apparatusaccording to claim 1, wherein the deflector deflects at least part ofthe guided electromagnetic waves so as to determine a coupling of theguided waves from the waveguide to the waveguide applicator.
 3. Anapparatus according to claim 1, wherein the deflector has a thickness inthe interval [λ/₂₀; λ/₁₀] in a direction normal to the plane of thedeflector.
 4. An apparatus according to claim 1, wherein the deflectoris shaped like an ellipse having a major principal axis a and a minorprincipal axis b.
 5. An apparatus according to claim 1, wherein thedeflector is shaped like a trapezium, such as a rectangle having a widtha and a height b.
 6. An apparatus according to claim 1, furthercomprising a member of a material having a relative permittivity largerthan 5, such as larger than 10, preferably larger than 25 positionedwithin the waveguide applicator for adjusting the resonance frequency ofthe cavity and/or the coupling of the guided waves between the waveguideand the waveguide applicator.
 7. An apparatus according to claim 6,wherein the material of the member comprises ceramic materialscomprising one or more materials selected from the group consisting ofAl₂O₃, TiO₂ or XTiO₃, where X is any group II element such as Ca or Mg.8. An apparatus according to claim 6, wherein the relative permittivityand/or the shape and/or the size of said member is chosen so as to makethe cavity resonant at a predetermined set of conditions such as thesample volume, the sample permittivity, and the coupling of the guidedwaves between the waveguide and the waveguide applicator.
 9. Anapparatus according to claim 1, further comprising means for adjustingthe position of the sample in the waveguide applicator in order toadjust the resonance frequency of the cavity and/or the coupling of theguided waves between the waveguide and the waveguide applicator.
 10. Anapparatus according to claim 9, further comprising supporting means forsupporting a container holding the sample, wherein the means foradjusting the position of the sample comprises means for adjusting asubstantially vertical position of said supporting means.
 11. Anapparatus according to claim 1, further comprising a first circulatorand a first dummy load, said first circulator being adapted to deflectat least part of electromagnetic radiation reflected from the applicatortowards the first dummy load.
 12. An apparatus according to claim 1,further comprising at least one power measuring means being adapted tomeasure power of at least part of the electromagnetic radiationdeflected from the first circulator.
 13. An apparatus according to claim12 further comprising a first memory means for storing information fromthe at least one power measuring means.
 14. An apparatus according toclaim 1, wherein the generating means comprises a magnetron.
 15. Anapparatus according to claim 1, wherein the generating means comprises asemiconductor based generator and a semiconductor based amplifier. 16.An apparatus according to claim 15, wherein the semiconductor basedamplifier comprises one or more silicon-carbide power transistors. 17.An apparatus according to claim 1, further comprising a thermalradiation sensitive element positioned so as to receive thermalradiation emanating from the sample.
 18. An apparatus according to claim17, wherein the thermal radiation sensitive element is adapted todetermine a temperature of the sample.
 19. An apparatus according toclaim 1, wherein the applicator comprises a protection screen forseparating the deflector and the waveguide from the sample, said screenbeing substantially transparent to the electromagnetic waves guidedtowards the waveguide applicator.
 20. An apparatus according to claim19, wherein said substantially transparent screen comprises one or moreof the materials selected from the group consisting of: PTFE (Teflon®)TPX, polypropene or polyphenylidenesulphide (PPS, Ryton®).
 21. Anapparatus according to claim 1, wherein the applicator comprises a drainfor draining sample from within the applicator.
 22. An apparatusaccording to claim 1, wherein the electromagnetic waves comprisesmicrowaves having a frequency in the interval 300 MHz - 300 GHz.
 23. Amethod for heating a sample, said method comprising the steps of: I.providing a heating apparatus according to claim 1 and inserting thesample in the applicator, II. generating electromagnetic radiation at afirst output power level, III. rotating the deflector for adjusting thecoupling factor between the waveguide and the resonant cavity.
 24. Amethod according to claim 23, wherein the sample has a first temperatureT₁, the method further comprising the steps of: heating the sample toobtain a second temperature T₂>T₁, rotating the deflector for adjustingthe coupling factor between the waveguide and the resonant cavity inresponse to the variation in the dielectric properties ε_(sample) of thesample.
 25. A method according to claim 23, Wherein step IlIl comprisesthe steps of: IV. performing the following steps one or more times:positioning the deflector in a first position and measuring a firstpower of electromagnetic radiation reflected from the waveguideapplicator, the reflected radiation corresponding to said first positionof the deflector, rotating the deflector to a second position that isdifferent from the first position and measuring a second power ofelectromagnetic radiation reflected from the waveguide applicator, thereflected radiation corresponding to said second position of thedeflector, and V. determining a preferred position of the deflectorbased on the amount of power reflected from the waveguide applicator inat least the first and second position.
 26. A method according to claim25, further comprising the steps of: VI. providing a first storingmeans, VII. storing information relating to the first position in thestoring means and storing the measured first power in relation thereto,and VIII. storing information relating to the second position in thestoring means and storing the measured second power in relation thereto.27. A method according to claim 26, wherein step V comprises processingthe stored measured powers for determining the preferred position of thedeflector corresponding to a local or absolute minimum in the measuredpower or to a predetermined ratio of the measured power to the firstoutput power level.
 28. A method according to claim 25, furthercomprising the steps of positioning the deflector in the preferredposition.
 29. A method according to claim 25, further comprising thesteps of positioning the deflector in the preferred position andgenerating electromagnetic radiation at a second output power levelwhich is larger than the first output power level.
 30. A methodaccording to claim 26, further comprising the steps of determining ameasure of the relative permittivity of the sample by comparing thestored measured powers with corresponding stored measured powers from adifferent sample.
 31. A method according to claim 26, further comprisingthe steps of determining an indication of the chemical composition ofthe sample by comparing the stored measured powers with correspondingstored measured powers from a sample of known chemical composition. 32.A method according to claim 31, wherein the sample comprises at leastone reactant for performing a chemical reaction, the method furthercomprising the steps of: performing the chemical reaction with the atleast one reactant, and determining a degree of reaction for thechemical reaction using the indication of the chemical composition ofthe sample.